HC37+38: Pharmacology: immunosuppression

What is immunosuppression?

Immunosuppressive drugs are drugs that lower the body’s normal immune response → they interfere with the immune system homeostasis. They are a variety of drugs that prevent the production of antibodies. Commonly, they are used to prevent rejection of the recipient’s body of an organ transplanted from a donor.

Immunosuppression is needed in case of:

• Autoimmune disease
• Transplantation

Examples of such cases are acute Graft versus Host disease and systemic lupus erythematosus. Glucocorticosteroids are examples of immunosuppressive drugs.

History

The history of immunosuppression is a successful story:

1. 1954: the first successful renal transplant
   • Identical twin donor without immunosuppression
2. 1959: the first successful allograft
   • Non-identical twin
   • Sublethal body irradiation
3. 1962: the first successful allograft with an unrelated donor
   • Azathioprine
   • > 1 year survival
4. 1963: reversal of rejection with steroids
5. 1967: the first heart transplant
   • Died of rejection in several days

Immunosuppressive theory

In reality, transplantation trades 1 set of problems for the other. Immunosuppressive drugs shift balances → new problems arise. Transplantation-patients have a multi-drug therapy of drugs with a low therapeutic index which very susceptible to side effects. Multidrug regimens allow for lower doses of each drug → minimize toxicity while providing adequate immunosuppression. They work at different signals/pathways of immune activation.

Acquiring the desired effect is always paired with toxic effects. Every drug has a certain toxicity:

• NIT: non-immune toxicity
  • Give side effects like nephrotoxicity
    • Side effects that aren’t immune related but organ related
  • A drug doesn’t affect primary action mechanisms
• ISE: immunosuppressive effect
  • Effective level
  • Risk of infections is higher
• ID: immunodeficiency
  • Infections, hospitalization

NIT and ID can overlap:

• Positive effects have a trade-off
• There already is toxicity at low levels

For this reason, drug concentrations have to be measured continuously → TDM (therapeutic drug monitoring) is necessary. Blood samples are continuously taken to measure the drug concentration.

Goals of immunosuppression

Goals of immunosuppression are:

• Providing adequate immunosuppression
• Minimizing adverse effects
• Treating adverse effects and chronic, drug-related problems
• Screening for drug-related complications

Current drugs are good for preventing acute rejections, but not chronic antibody mediated rejections → there is a recent improvement in short-term outcomes but less improvement in long-term outcomes. Rejections are often chronic.

The focus is on different combinations:

• Reducing doses of each individual drug to improve outcomes
• Combining of multiple, low-dose drugs is more effective than 1 single drug in its optimal dose

Achieving immunosuppression:

Immunosuppression can be achieved by blocking the immune system. The adaptive immune system can be targeted at multiple sites:

• Belatacept: blocks direct interaction between antigen presenting cells and T-cells
• Cyclosporin and tacrolimus: inhibit T-cell proliferation and block downstream signaling → less IL-2 production
• Rapamycin and MMF/azathioprine → inhibit proliferation of activated T-cells

Classification

Immunosuppressants can be classified in 3 groups:

• Inhibitors of cytokine production
  • Calcineurin inhibitors
    • Cyclosporine
    • Tacrolimus
• Inhibitors of cytokine gene expression
  • Corticosteroids
    • Prednisolone
• Cytotoxic drugs
  • Antimetabolites
    • Azathioprine, MTX, mycophenolate
  • Alkylating agents
    • Cyclophosphamide

Inhibitors of cytokine production

Cyclosporin and tacrolimus are calcineurin inhibitors → they inhibit downstream signaling. They are inhibitors of cytokine production. Their action mechanism lies in the T-cells:

1. An antigen presenting cell binds to the T-cell
2. Calcineurin inhibitors inhibit the mediator calcineurin → the T-cell cannot be activated
   • IL-2 normally is a major driver of this → calcineurin inhibitors inhibit IL-2

Cyclosporines:

Cyclosporines are classical drugs. Cyclosporine A is a peptide derived from the fungus tolypocladium inflatum. There are multiple formulations:

• Oil-based
  • Variable absorption
• Micro-emulsion
  • Preferred

Side effects of cyclosporine are:

• Renal insufficiency
• Hypokalemia/hypomagnesemia
• Hyperuricemia
• Neurotoxicity
  • Encephalopathy
  • Seizures
  • Tremors
  • Neuropathy
• Gingival hyperplasia
• Hirsutism
• Et cetera

Tacrolimus:

Tacrolimus is a more recent drug, but not new either. It is a macrolide derived from the fungus streptomyces tsukabaensis. It is one of the most widely used immunosuppressants, for example after kidney transplantations.

Tacrolimus interacts with substances that potentiate and inhibit CYP450 → drug-drug interactions:

• Inhibit CYP450
  • Azoles
  • Calcium channel blockers
  • Amiodarone
  • Mycins
  • Metronidazole
  • Grapefruit
• Potentiate CYP450
  • Rifampicin
  • Phenytoin
  • St. John’s Wort

Side effects of tacrolimus are:

• Renal insufficiency
  • Kidney function has to be monitored
• Diabetes
  • The chance of developing diabetes is larger than when using cyclosporin
• Dyslipidemia
• Hypomagnesemia/hyperkalemia
  • Changes calcium influx affects the homeostasis of all ions

Inhibitors of cytokine gene expression

Mechanism:

Corticosteroids influence various parts of the inflammation cascade:

1. Glucocorticoids can diffuse through the cell membrane because they are very lipophilic
2. Glucocorticoids bind to glucocorticoid receptors
   • These are nuclear receptors in the cytosol of almost every cell
   • They are not in the nucleus
   • Heat shock proteins (HSP) and immunophilin (IP) are involved
3. Glucocorticoids go to the nucleus
4. Glucocorticoids regulate the transcription of:
   • Expression of anti-inflammatory mediators
   • Repression of pro-inflammatory mediators

Because transcription takes place, it takes several hours to days until the effect of the glucocorticoids is noticeable.

Arachidonic acid pathway:

Corticosteroids also inhibit eicosanoid production. It affects the arachidonic acid pathway. Eicosanoid is an arachidonic acid. This is also done by NSAIDs → have the same side effects as corticosteroids. The lipid pathway and the COX-pathway are inhibited → less vasodilation and hypertension.

Responsiveness:

Approximately 50% of patients respond to glucocorticoid therapy. Non-responsiveness is a negative prognostic factor. Factors affecting glucocorticoid response are mainly genetic:

• Single nucleotide polymorphisms (SNPs) in the glucocorticoid receptor gene or major regulatory genes
• Difference in gene expression

However, this doesn’t explain the full variation in response.

Side effects:

Corticoid therapy is in fact the purposely unbalancing of endogenous levels of steroids. Because receptors for glucocorticoids are present throughout the entire body, the drug is associated with many side effects. The main side effects are:

• Cushing’s disease
• Osteoporosis
• Myopathy
• Glucose intolerance → diabetes
• Infections
• Ulcers in the GIT
• Endocrine problems
• Neuronal problems
• Cardiovascular problems
• Eye problems
• Skin problems

It only is necessary to remember 3 or 4 side effects.

Cushing’s disease:

Cushing’s disease can be caused by prescribing too many corticosteroids. The main symptoms are:

• Osteoporosis
• Obesity
• Hypertension
• Skin atrophy
• Striae

Addison’s disease:

Cushing’s disease technically is the opposite of Addison’s disease, which can be caused by low levels of corticosteroids. The main symptoms are:

• Nausea
• Headache
• Fever
• Hypoglycemia
• Hyperkaliemia
  • Glucocorticoids also bind to mineralocorticoid receptors → affects kalium levels
• Hypotension

Prednisolone:

Prednisone is the pro-drug of prednisolone. A prodrug is an inactive drug which needs metabolism to become active. Once prednisone is the body, it is rapidly converted into prednisolone → its active form.

Prednisolone therapy leads to decrease in cortisol. It is a non-specific, anti-inflammatory glucocorticoid which interrupts multiple steps in the immune activation. It is highly effective for prevention of rejection, for example in Guest versus host disease, SLE, psoriasis and cancer treatment. However, it has many long-term adverse effects.

Cytotoxic drugs

Cytotoxic drugs prevent lymphocyte proliferation. There are 2 groups of cytotoxic drugs:

• Anti-metabolites
• Alkylating agents

Azathiroprine and MMF are prodrugs that inhibit IMP, an antimetabolite. Both drugs non-competitively inhibit de novo purine synthesis. They work complementary to other, more upstream, approaches.

Azathioprine:

Azathioprine is a pro-drug that is activated into its active form, 6-mercaptopurine, in the body. It is an older antiproliferative agent. Side effects are:

• Bone marrow suppression
• Malignancies

Mycophenolate Mofetil:

Mycophenolate Mofetil (MMF) is a prodrug hydrolyzed into mycophenolic acid, its active form. It is a more recent agent that is now preferred to azathioprine. Side effects are:

• Gastro-intestinal
  • Nausea
  • Gastritis
  • Diarrhea
• Leukopenia and thrombocytopenia
  • Dose-related

mTOR inhibitors:

mTOR inhibitors are proliferation signal inhibitors. They are active in T-cells and inhibit the enzyme mTOR → proliferation of T-cells is blocked. This arrests further cell cycle, there’s no further lymphocyte proliferation.

Examples of mTOR inhibitors are:

• Everolimus
  • Analogue of sirolimus
  • Approved for renal transplants
  • Side effects:
    • Oral ulcers
    • Dyslipidemia
    • Bone marrow suppression
      • Anemia
      • Thrombocytopenia
    • Potentiates Cl^- nephrotoxicity
• Sirolimus

Standard immunosuppressive regimen

The immunosuppressants usually prescribed are a combination of:

• Calcineuron inhibitors
  • Cyclosporin
  • Tacrolimus
• Anti-metabolite
  • Azathioprine
  • Mycophenolate mofetil
• Steroids

Thus, a combination of drugs with different pathways is prescribed. Doses of each drug are low to minimize toxicity. Calcineurin inhibitors are the most effective.

Standard regimens change over time. Currently, the most used standard regimens are:

• 49% of cases: tacrolimus/steroid/MMF
• 28,5% of cases: cyclosporin/steroid/MMF
• 3,8% of cases: tacrolimus/MMF
• 1,9% of cases: tacrolimus/steroid

In conclusion, monotherapy and even combinations of 2 drugs are hardly prescribed. The benefits of giving 3 low doses outweigh the side effects of a heavy dose of 1 drug.

Practical considerations:

Practical considerations to take into account are:

• Tacrolimus
  • Slow uptitration
    • Tapering to reduce withdrawal symptoms
  • Rapid metabolizers
• Mycophenolate
  • Can be taken with food/meds
  • GI symptoms respond to change in dose